Process for blocking open hearth heats



Patented Oct. 24, 1950 2,527,186 PROCESS FOR BLOCKING OPEN HEARTHFrederick J. Griifiths, Massillon, Ohio, assignor to Chromium Mining andsmelting Corporation, Limited, Sault Ste. Marie, Ontario, Quebec,Canada, a corporation of Canada No Drawing. Application April 26, 1946,Serial No. 665,349

. 6 Claims.

This invention relates to metallurgy and has for an object the provisionof an improvedmetallurgical method or process. More particularly, theinvention contemplates the provision of an improved method or processfor the manufacture of steel in the open hearth furnace. A furtherobject of the invention is to provide an improved method or process forblocking open hearth furnace heats to retard or inhibit carbon loss.

Steel products are produced to a large extent by means of open hearthfurnace processes. An open hearth process comprises a refining treatmentof a batch or bath of molten metal which may be preceded by a meltingtreatment involving the melting of scrap iron or steel or both. Eachbatch or bath of metal treated is called a heat. The refining treatmentusually involves carbon elimination and may involve the elimination ofsuch other elements as silicon, phosphorus and sulphur. Elimination ofthese elements is accomplished by subjecting the metal to the action ofbasic material like lime and oxidizing material like iron oxide. Thelime and iron oxide and substances like silica, which'may be present inthe furnace charge or which may be formed by oxidation of siliconpresent in thecharge or both, form a slag Which ultimately provides amolten covering layer for the metal undergoing refining.

Iron oxide is maintained in the slag in substantial concentrationthroughout the refining period and some iron oxide dissolves in themetal. The iron oxide contained in the slag and in'the metal reacts withcarbon contained in the metal and effects its elimination as a gaseousoxide of carbon.

After refining has been completed, it'iscustomary to add deoxidizingagents or alloying' ele-' ments or both. Deoxidizing agents and alloyingelements usually are added when the carbon content of the metal has beenreduced to about the amount which it is desired to retain in thefinished product, and, therefore, steps must be taken to prevent furthercarbon elimination. Carbon elimination is retarded or prevented bypreventing further reaction of iron oxide contained in the metal andslag with carbon contained in the metal. This procedure is calledblocking the heat, and it is accomplished, usually, by providing siliconfor reaction with the iron oxide in place of carbon, since siliconreacts preferentially with iron oxide. 1

According to some heretofore customary practices, silicon has beenprovided by adding lumps of low-silicon ferro-silicon to the moltenmetal baths. Ferrosilicon added to a bath is melted and dispersedtherein, and the dispersed silicon-of the ferrosilicon reactspreferentially With'iron oxide dissolved in the bath, preventingreaction of the iron oxide with carbon contained in the under the tradename SIL-X ;:1ooo I7 Example III n Parts Sodium nitrate 217'Fe-rrosilicon (57%"Si) (sold in commerce.

2 bath. Silicon dispersed in the bath reacts with iron oxide at theslag-metal interface to some extent, also, further reducing thepossibility of iron oxide reacting with carbon of the bath.

Some disadvantages are attached to the practice of incorporating siliconin a bath of molten' steel by adding a silicon alloythereto. Thus, for,

example, considerable time may berequire'd to eifect melting of thealloy and dispersion of the silicon, and some silicates formed byreaction of the silicon With iron oxide may be retained and formundesirable inclusions in the steeL' The present invention is based onmy discovery that the addition to a basic slag in an open hearth steelfurnace'at the conclusion of the carbon elimination stage of anexothermic reaction mix! ture comprising a non-carbonaceous reducingagent, such as silicon or aluminum, and an oxidizing agent, like analkali metal nitrate (sodiumnitrate) or an alkali metal chlorate (sodiumchlorate), results in modification of the characteristics of the slagand. inhibition of the reaction 'betweencarbon contained in the steeland iron oxide contained in the steel and slag.

According to the preferred method of them-' vention, thenon-carbonaceous reducing agent and the oxidizing agent are employed in"the form of an intimate admixture capable of reaction exothermicallyupon ignition to generatea substantial quantity of heat. A preferredexothermic reaction mixture employed in carrying out a method or processof the invention is one comprising, or even consisting essentially of,ferrosilicon and sodium nitrate in'which the ferrosilicon is present insuch amount and proportion I as to provide silicon for reaction with allof the sodium nitrate and with iron oxide and calcium oxide of the slagwith the stantial quantity of heat.

The"f0110wing examples illustrate types of re action mixtures which havebeen employed effectively in carrying out a method or process of theinvention (proportions are given in parts by weight) Example I V PartsSodium nitrate .14'5 Ferrosilicon (57% Si) (sold in commerce under thetrade name SIL-X 1000 Example II e V Parts Sodium nitrate '75Ferrosilicon (57% Si) (sold in commerce under the trade name S IL-X-217)"1000 production 'ofa subi Reaction mixtures of the types illustratedabove preferably are employed in the form of small agglomerates in whichthe ferrosilicon is present in the form of small particles (preferablyminus 65-mesh) which are intimately associated with andbonded togetherby means of the sodium nitrate. Agglomeration is effected by mixing theparticles of ferrosilicon and finely divided sodium nitrate intimatelywhile moistened with water in amount equal in Weight to about two tothree percent of the weight of the mixture, molding the resultingplastic mixture into masses of suitable sizes, heating the massescarefully to drive ofi water and melt the sodium nitrate in place, andcooling to efiect solidification and crystallization of the sodiumnitrate.

At the conclusion of the refining stage, when the carbon content hasbeen reduced to the desired concentration and, the steel is ready forthe addition of deoxidizers or alloying elements orboth, theslag-forming components have been well digested or reacted and thereexists a well- ,shaped, creamy, mature slag which is characterized by amoderate, uniform boiling action over the entire surface. At this time,iron oxide of the slag may be passing slowly from the slag to the metal,as the result of reaction of the iron oxide dissolved inthe metal withthe carbon of the metal, in order constantly to re-establish equilbriumbetween iron oxide in the metal and iron oxide in the slag in accordancewith the relative solubilities of iron oxide in the metal and slagphases.

The addition to a well-shaped slag of a quantity of exothermicreaction'mixture of the type I. illustrated above efiectively retardsoxidation of the carbon of the steel and permits the addition Ofdeoxidizers and alloying elements to be made and tapping of the heat tobe carried out in a shorter period of time and with the production of asmaller amount of scrap than when oxidation of carbon is retarded byadding a low-silicon alloy to the bath.

The following example illustrates results obtained in employingprocesses of the invention using exothermic reaction mixtures of thetypes illustrated above:

Heat No. g'rigg Scrap Minutes Pounds Total 27 Heats 031 144, 030 Averageper Heat 23. 4 5, 331 2. 3%

In the processes of the invention for which the data are given above,the average time from of scrap produced was 6.3% of the weight of themetal.

When added to molten slag 0n the surface of a molten bath of metal, anexothermic reaction mixture of the type illustrated above becomesignited, the sodium nitrate reacts with a portion of the silicon of theferrosilicon generating heat which efiects a local temperature rise inthe slag and melts the remainder of the ferrosilicon. The silicon of themolten ferrosilicon reacts rapidly with the iron oxide of the slag, asthe result of the heat generated and the increased temperature,disturbing the equilibrium between the iron oxide dissolved in the metaland the iron oxide dissolved in the slag and preventing theflow of ironoxide from the slag to the metal. Another result of reaction of thecomponents of the exothermic reaction mixture is the production of analkali metal oxide (sodium oxide) which tends to combine chemically withiron oxide of the slag, or, in other words, increases the capacity ofthe slag to retain iron oxide and prevent its transfer to the metal.

Exothermic reaction mixtures are employed in processes of the inventionin relatively small amounts. The non-carbonaceous reducing agent, suchas silicon, in the added reaction mixture preferably is present inamount sufficient to react with all of the oxidizing material of themixture and with iron oxide of the slag and generate a substantialquantity of heat but insufficient to penetratethe slag and enter themolten steel in substantial amount. Normally, amounts of exothermicreaction mixtures equal in weight to about one to three percent (1.0 to3.0%) of the weight of the slag may be employed satisfac torily incarrying out a method or process of the invention. Any suitable amountsof exothermic reaction mixtures may be employed in carrying out a methodof the invention.

I claim:

1. In a method of producing steel in an open hearth furnace in which amolten bath of metal is subjected to oxidation during a refining periodbeneath and in contact with a basic slag containing calcium oxide andiron oxide to efiect carbon oxidation and the production of a moltenmetal product containing carbon in predetermined amount and oxidation ofcarbon thereafter is retarded, the improvement which comprises ignitingin contact with the slag after completion of the refining period anexothermic reaction mixture comprising oxidizing material selected fromthe group consisting of alkali metal nitrates and alkali metalchlorates, and noncarbonaceous reducing material comprising an elementof the group consisting of silicon and aluminum in amount sufficient toreact with all of the oxidizing material of the mixture and with ironoxide of the slag and generate a substantial quantity of heat but notsufiicient to penetrate the slag and enter the molten metal insubstantial amount, thereby to increase the temperature of the slag,promote reaction between the iron oxide of the slag and thenon-carbonaceous reducing material and retard reaction between ironoxide contained in the slag and carbon contained in the metal.

2. In a method of producing steel in an open nitrates and alkali metalchlorates, and material containing elemental silicon in amountsufficient to react with all of the oxidizing material of the mixtureand with iron oxide contained in the slag and generate a substantialquantity of heat but not suificient to penetrate the slag and enter themolten metal in substantial amount, thereby to increase the temperatureof the slag, promote reaction between the iron oxide of the slag and thesilicon-containing material and retard reaction between iron oxide ofthe slag and carbon contained in the metal.

3. In a method of producing steel in an open hearth furnace in which amolten bath of metal is subjected to oxidation during a refining periodbeneath and in contact with a basic slag containing calcium oxide andiron oxide to eiiect carbon oxidation and the production of a moltenmetal product containing carbon in predetermined amount and oxidation ofcarbon thereafter is retarded, the improvement which comprises ignitingin contact with the slag after completion of the refining period anexothermic reaction mixture comprising sodium nitrate and materialcontaining elemental silicon in amount suificient to react with all ofthe sodium nitrate of the mixture and with iron oxide contained in theslag and generate a substantial quantity of heat but not suflicient topenetrate the slag and enter the molten metal in substantial amount,thereby to increase the temperature of the slag, promote reactionbetween the iron oxide of the slag and the silicon-containing materialand retard reaction between iron oxide of the slag and carbon containedin the metal.

4. In a method of producing steel in an open hearth furnace in which amolten bath of metal is subjected to oxidation during a refining periodbeneath and in contact with a basic slag containing calcium oxide andiron oxide to efiect carbon oxidation and the production of a moltenmetal product containing carbon in predetermined amount and oxidation ofcarbon thereafter is retarded, the improvement which comprises ignitingin contact with the slag after completion of the refining period anexothermic reaction mixture comprising sodium nitrate and ferrosiliconin amount sufiicient 'to react with all of the sodium nitrate of themixture and with iron oxide contained in the slag and generate asubstantial quantity of heat but not sufficient to penetrate the slagand enter the molten metal in substantial amount, thereby to increasethe temperature of the slag, promote reaction between the ironoxide ofthe slag and the ferrosilicon and retard reaction between iron oxide ofthe slag and carbon contained in the metal.

5. In the production of steel in a basic open hearth furnace process,the improvement which comprises igniting in contact with the slag aftercompletion of the refinin period an exothermic reaction mixturecomprising sodium nitrate and material containing elemental silicon inamount sufficient to react with all of the sodium nitrate of the mixtureand with iron oxide contained in the slag and generate a substantialquantity of heat but not suflicient to penetrate the slag and enter themolten metal in substantial amount, thereby to increase the temperatureof the slag,

promote reaction between the iron oxide of theslag and thesilicon-containing material andretard reaction between iron oxide of theSlag and carbon contained in the metal.

6. In the production of steel in a basic open 7 hearth furnace process,the improvement which comprises igniting in contact with the slag aftercompletion of the refining period an exothermic reaction mixturecomprising, sodium nitrate and ferrosilicon in amount sufficient toreact with all of the sodium nitrate of the mixture and with iron oxidecontained in the slag and generate a substantial quantity of heat butnot sufiicient to penetrate the slag and enter the molten metal insubstantial amount, thereby to increase the temperature of the slag,promote reaction between the iron oxide and the ferrosilicon and retardreaction between iron oxide of the slag and carbon contained in themetal.

FREDERICK J. GRIFFITHS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

5. IN THE PRODUCTION OF STEEL IN A BASIC OPEN HEARTH FURNACE PROCESS,THE IMPROVEMENT WHICH COMPRISES IGNITING IN CONTACT WITH THE SLAG AFTERCOMPLETION OF THE REFINING PERIOD AN EXOTHERMIC REACTION MIXTURECOMPRISING SODIUM NITRATE AND MATERIAL CONTAINING ELEMENTAL SILICON INAMOUNT SUFFICIENT TO REACT WITH ALL OF THE SODIUM NITRATE OF THE MIXTUREAND WITH IRON OXIDE CONTAINED IN THE SLAG AND GENERATE A SUBSTANTIALQUANTITY OF HEAT BUT NOT SUFFICIENT TO PENETRATE THE SLAG AND ENTER THEMOLTEN METAL IN SUBSTANTIAL AMOUNT, THEREBY TO INCREASE THE TEMPERATUREOF THE SLAG, PROMOTE REACTION BETWEEN THE IRON OXIDE OF THE SLAG AND THESILICON-CONTAINING MATERIAL AND RETARD REACTION BETWEEN IRON OXIDE OFTHE SLAG AND CARBON CONTAINED IN THE METAL.